<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <title>Document</title>
</head>

<body>

    <script type="text/javascript">
        // then参数中的函数返回值

        function queryData(url) {
            return new Promise(function(resolve, reject) {
                var xhr = new XMLHttpRequest();
                xhr.onreadystatechange = function() {
                    if (xhr.readyState != 4) return;
                    if (xhr.readyState == 4 && xhr.status == 200) {
                        // 处理正常的情况
                        resolve(xhr.responseText);
                    } else {
                        // 处理异常情况
                        reject('服务器错误');
                    }
                };
                xhr.open('get', url);
                xhr.send(null);
            });
        }

        queryData('http://localhost:3000/data')
            .then(function(data) {
                return queryData('http://localhost:3000/data1');
            })
            .then(function(data) {
                return new Promise(function(resolve, reject) {
                    setTimeout(function() {
                        resolve(123);
                    }, 1000)
                });
            })
            .then(function(data) {
                return 'hello';
            })
            .then(function(data) {
                console.log(data)
            })
    </script>
</body>

</html>